Present aerospace drive systems which rely on mechanical devices for torque transmission have a few drawbacks. Mechanical failures can occur due to torque overloads and the driven component cannot be isolated from the drive system. Magnetic couplings, on the other hand, provide torque transmission without mechanical contact and losses. Magnetic couplings are especially well suited for use in isolated drive systems because they eliminate the problems associated with rotating shaft seals, namely, inherent leakage and friction. A magnetic coupling also will slip when excessive torque occurs, thus preventing mechanical failure from the torque overloads.
One type of magnetic coupling, a radial type permanent magnet coupling, is made up of an inner and an outer member which are rotatable with respect to one another. Each member includes a cylindrically shaped iron core with magnets mounted to one of its surfaces. Typically, the magnets have a radial direction of magnetization. In general, a containment ring is placed over the magnets located at the outer surface of the inner member, to contain the permanent magnets against centrifugal force during rotation. The radial thickness of the containment ring is dictated by the maximum operating speed of the magnetic coupling and increases as a function of speed. The radial distance between the inner and outer magnets mounted on the inner and outer member, defines the "magnetic" air gap. An increase in the "magnetic" air gap causes a proportional decrease in the magnetic flux that flows between the inner and the outer members. With a decrease in flux, the coupling will transmit less torque because torque transmission is proportional to the squared value of the magnetic flux. Since the containment ring is located between the inner and outer magnets, an increase in the size of the containment ring causes a decrease in the torque transmission capability. In the case of high speed applications, such as aircraft cooling systems utilizing magnetic couplings within turbo compressors, the containment ring becomes prohibitively thick, thus limiting the application of magnetic couplings.
The present invention is directed at reducing the physical size of the coupling by eliminating the containment ring or by keeping its radial thickness to a minimum. That, in turn, will decrease the "magnetic" air gap which, in turn, will allow a coupling of a given size to transmit more torque.